Silicon oxidation plays an important role in the manufacturing of semiconductor devices. The thermal oxidation of silicon produces a silicon dioxide (SiO.sub.2) passivation film with excellent electrical properties and is thus an important processing step during wafer fabrication. There are two known methods for thermal oxidation. The first is a standard 1 atmosphere (atm) oxidation using steam as the oxidant. This process requires a high processing temperature, in excess of 1000.degree. C., and a long oxidation time, approximately 5 hours, for an oxidation thickness of 1 .mu.m (10,000 .ANG.). An inherent problem with the conventional 1 atm oxidation process is the thermal budget management for sub-micron devices. The thermal budget is a function of the oxidation time and temperature, a 1 atm oxidation process may result in the semiconductor device exceeding its thermal budget.
A second method uses high pressure to accelerate the oxide growth on the silicon substrate. The high pressure oxidation (HiPOx) process utilizes either pure steam or dry oxygen as the oxidant. One advantage of using elevated pressures is that the processing temperature may be lowered as compared to the conventional 1 atm oxidation process. In the pure steam HiPOX process, the oxidation rate increases parabolicly with increased pressure. As can be seen in FIG. 1, the oxidation time, at a same temperature, for a 1 .mu.m thick oxide in a conventional 1 atm steam process is an order of magnitude larger than oxidation times for either a 10 atm or 25 atm pure steam HiPOx process.
A problem with the pure steam HiPOx process is that the oxidation time is too fast when the desired oxide thickness is less than 1 .mu.m. The process becomes unmanufacturable and uncontrollable because the oxidation completes during the transient or ramp-up time of the HiPOx equipment. It is thus, not possible, to control either the desired oxide thickness or the material properties of the oxide. A second problem with the pure steam HiPOx process is that dirty particles are generated during the oxidation. The particles are caused by the superheated steam impinging on the quartz process tube or other parts made of quart inside the equipment. The generated particles increase film defectivity which is detrimental to semiconductor devices, and thus decrease device yield. A third problem with the pure steam HiPOx process is the Kooi effect typically observed in LOCOS isolation. In this instance, the adjacent underlying substrate is a masking nitride instead of silicon. The steam diffuses through the nitride to form ammonia at the nitride / pad oxide interface during the oxidation process. Then the ammonia diffuses through the pad oxide to form silicon nitride spots at the pad oxide / silicon interface resulting in a pitted active region which causes a device reliability problem.
An alternative HiPOx process utilizes dry oxygen as the oxidant. The dry oxygen process poses no dirty particle generation problem but the oxide growth rate is 2 orders of magnitude slower than that of the pure steam process, as illustrated in FIG. 2. While this dry oxygen HiPOx process is controllable because the oxidation completes after the equipment has reached steady state operation conditions, the process offers no gain in the thermal budget due to the extremely long process time. In fact, the dry oxygen HiPOx process takes the same amount of time to grow a given oxide thickness as a conventional 1 atm steam oxidation process. So while the processing temperature may be lowered slightly by going to high pressure in a dry oxygen environment, no advantage in processing time is gained.
Thus, it is desirable to find a dean oxidation process which offers a low thermal budget.